Pharmacological Modulation of 5-HT_2C Receptor Activity Produces Bidirectional Changes in Locomotor Activity,...

Converging lines of behavioral, electrophysiological, and biochemical evidence suggest that 5-HT_2C receptor signaling may bidirectionally influence reward-related behavior through an interaction with the mesolimbic dopamine (DA) system. Here we directly test this hypothesis by examining how modulating 5-HT_2C receptor activity affects DA-dependent behaviors and relate these effects to changes in nucleus accumbens (NAc) DA release. In C57BL/6 mice, locomotor activity and responding for a conditioned reinforcer (CRf), a measure of incentive motivation, were examined following treatment with three 5-HT_2C receptor ligands: the agonist CP809101 (0.25-3 mg/kg), the antagonist SB242084 (0.25-1 mg/kg), or the antagonist/inverse agonist SB206553 (1-5 mg/kg). We further tested whether doses of these compounds that changed locomotor activity and responding for a CRf (1 mg/kg CP809101, 0.5 mg/kg SB242084, or 2.5 mg/kg SB206553) also altered NAc DA release using in vivo microdialysis in anesthetized mice. CP809101 reduced locomotor activity, responding for a CRf, and NAc DA release. In contrast, both SB242084 and SB206553 enhanced locomotor activity, responding for a CRf, and NAc DA release, although higher doses of SB206553 produced opposite behavioral effects. Pretreatment with the non-selective DA receptor antagonist α-flupenthixol prevented SB242084 from enhancing responding for a CRf. Thus blocking tonic 5-HT_2C receptor signaling can release serotonergic inhibition of mesolimbic DA activity and enhance reward-related behavior. The observed bidirectional effects of 5-HT_2C receptor ligands may have important implications when considering the 5-HT_2C receptor as a therapeutic target for psychiatric disorders, particularly those presenting with motivational dysfunctions.